Method of evaporating liquid refrigerant in a semi-flooded type evaporator



Feb. 28, 1967 P. A. WELLER 3,306,063

METHOD OF EVAPORATING LIQUID REFRIGERANT IN A SEMI-FLOODED TYPEEVAPORATOR Original Filed 001;. 5, 1962 2 sheets-sheet 1 I NVENTOR.

PETE/a QEL L i BY Feb. 28, 1967 METHOD OF EVAP TING LIQUID REFRIGERANTIN 2 Sheets-Sheet 2 Original Filed Oct.

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. 4 g 8 w w m m4 7 e w w W. \w m u \x R Q. Q \m R\ Q. NM \NMX w fl &\ WQ o m Q Q 6 Q 1 s .0 m m w y Q b F. :U k Hi Q NM MM -m Q. \W I I I N? Im a F 3. a m b L United States Patent 3,396,063 METHOD OF EVAPORATENGLIQUID REFRIGERANT IN A SEMI-FLOODED TYPE EVAPDRATGR Peter A. Weller,Farniington, Mich, assignor to American Radiator & Standard SanitaryCorporation, New York, N.Y., a corporation of Delaware Originalappiication Get. 3, i962, Ser. No. 228,055, now Patent No. 3,240,265,dated Mar. 15, 1966. Divided and this application June 1, 1965, Ser. No.478,500

4 Claims. (Cl. 62-114) This invention relates to refrigerationevaporator systems, and more particularly to an improved refrigerationevaporator of the flooded type, and to the method of operation.

This application is a division of my copending applica tion, Serial No.228,055, filed October 3, 1962, now Patent Number 3,240,265.

In the operation of flooded evaporators in refrigeration systems, it iscommon practice to submerge only part of the tubes in liquidrefrigerant, and allow the boiling action of the refrigerant to splashliquid from a lower level over the upper tubes.

This, theoretically, is a good way to operate. However, as the loadreduces, i.e., as the heat exchange tubes operate at a lower temperaturelevel, the boiling action of the liquid refrigerant subsides because ofless temperature differential between the liquid refrigerant and theheat exchange tubes. The result is that there is a lesser amount ofupper tube wetting by the liquid because of the diminished boilingaction. As this happens, the effective heat transfer surface is reduced.As a consequence, the mean difference in temperature between theoutgoing water in the tubes and the boiling temperature of therefrigerant in the evaporator is not reduced as the load in reduced(this mean temperature difference being termed the approach). If thetube surfaces were more uniformly in operation under all loadconditions, the approach would be reduced with a reduction in the loadresulting in increased operating efiiciency of the system.

Accordingly, an important step forward would be provided in the art by anovel refrigerant evaporator wherein the tubes could be operated at lessthan fully submerged levels, wherein the boiling action was neverthelessmaintained at a uniform rate and thereby utilized to splash liquiduniformly over all of the exposed tubes. A further advance would beprovided in the art by a refrigeration evaporator wherein the boilingsection of the refrigerant were fully utilized and wherein otherimprovements of more efiicient operation were provided.

An important object of the present invention is to provide a method ofevaporating liquid refrigerant in a refrigeration evaporator of theheavy duty industrial semifiooded type.

A further object is to provide a method of evaporating liquidrefrigerant wherein the bubbling action due to flash vaporization of aportion of the entering liquid refrigerant is substantially equallydistributed throughout the evaporator to substantially evenly splashliquid over the heat exchange tubes that are exposed above the liquid.

Another object of the invention is to provide a method wherein a thinlayer of oil is resent on the pool of liquid refrigerant, circulationbeing provided within the evaporator to maintain the layer of oil evenlyover the surface of the pool of refrigerant.

Another object is to provide a method of evaporating liquid refrigerantfrom a shallow, horizontal pool of uniform depth on which a uniformcovering layer of oil is provided for improved heat transfer.

Other objects of this invention will appear in the following descriptionand appended claims, reference being had to the accompanying drawingsforming a part of this specification wherein like reference charactersdesignate corresponding parts in the several views.

In the drawings:

FIGURE 1 is a transverse sectional view of a refrigerant evaporator madein accordance with the present invention; and

FIGURE 2 is a longitudinal sectional view showing a schematicarrangement of a liquid refrigerant distribution system in an evaporatormade in accordance with the present invention.

Before explaining the present invention in detail, it is to beunderstood that the invention is not limited in its application to thedetails of construction and arrangement of parts illustrated in theaccompanying drawings, since the invention is capable of otherembodiments and of being practiced or carried out in various ways. Also,it is to be understood that the phraseology or terminology employedherein is for the purpose of description and not of limitation.

Briefly, the present invention relates to a novel and improvedrefrigeration evaporator of the semi-flooded type wherein the bubblingaction due to the liquid refrigerant which boils is fully utilized tosplash liquid over all of the heat exchange tubes, even when operatingwith a liquid level substantially below the top of the tube bundle.

The unexpected result is provided by a tube bundle which in transversecross section has a relatively shallow, even depth, and with vapordiverters strategically located in the bundle to force the rising vaporuniformly through the entire tube bundle for uniform tube wetting; andby a liquid refrigerant delivery system assuring uniform spreading ofthe entering liquid refrigerant over the bottom of the unit.

Still further, uniform and improved results of operation are provided inaccordance with the present invention by utilizing oil in the liquidrefrigerant to facilitate wetting of the heat exchange tubes bysplashing, even at low liquid levels, by retaining the oil in auniformly distributed layer over the top of the liquid refrigerant.

As shown in FIGURE 1, an improved refrigeration evaporator in accordancewith the present invention is designated by the reference numeral 18.This unit includes an upper dome-like shell of pressure resistantmaterial such as steel, and designated by the numeral 12. An outerinsulation layer is provided at 14. A flat transverse wall 16 isprovided across the bottom of the upper steel shell 12 to support thetube bundle as will be hereinafter set forth. The shell 12 and wall 15are joined by welding as at 17.

Across the bottom of the unit there is provided a concave wall 18, soconfigured in order to resist internal pressures of the unit and, aswill be evident from the following description, providing a vapor spacebeneath the Wall 16 for insulation, and for purposes of equalizingpressures above and below wall 16.

By reference to FIGURE 1, it will be noted that the tube bundle isdesignated broadly by the number 20 and is of generally rectilinearouter configuration. Actually, the tube bundle is rectangular in crosssection and the bounding sides, top and bottom, are generally straightlines. It will also be noted that the tubes 22 of the bundle 20 lie in agenerally straight line along the bottom, conforming to the fiat uppersurface of the intermediate or fiat transverse wall 16.

Thus, every vertical column of tubes designated by the reference numeral24 is essentially of the same height and thus operates withsubstantially the same submergence to obtain complete tube wetting.

By reference to both figures of the drawings, it will be noted that adistribution main tube designated by the reference numeral 26 extendsaxially of the unit for conducting liquid and flash gas from the highside float or other valve of a suitable refrigeration system. It shouldbe noted at this point that liquid refrigerant on the condenser side ofa float valve is at a higher temperature and higher pressure thanrefrigerant on the evaporator side of the float valve. As soon as liquidrefrigerant passes through the float valve, it is subjected to thetemperature and pressure conditions on the evaporator side. The lowertemperature and lower pressure on the evaporator side results in aportion of the liquid refrigerant which passes through the floatimmediately vaporizing. This is termed flash vaporization. Extendingdownwardly from the distributor main 26 are a plurality of feeder tubes28 of vertical disposition and uniformly spaced along the length of themain 26. Nozzles 30 are provided at the bottom end of each of thevertical feeder tubes 28 and are directed horizontally and sized toimpart a moderate velocity to the incoming refrigerant and are directedin the fanlike pattern shown by the arrows 32, FIGURE 2, to create auniform circulation across the bottom of each compartment 34, 36 and 38defined by the transverse end tube sheets 40 and the transverse tubesupport sheets 4-2.

At each support sheet 42, there are provided circulator tubes 44,comprising a down comer 46 and a lateral feed 48 with an upturned endelbow-nozzle 50. The circulator tubes 44 run from the distribution tube26 downward via the down comer 46, then horizontally via the lateral arm48 below the tube bundle to the outer edges, then rise and turn via theelbow-nozzle 50 towards the tube support sheets 42, in the manner shown,so as to impart a jet of refrigerant through Openings 52 at the lowercorners of the tube support sheets 42. These jets are arranged to effectthe circulation between compartments in the same direction of rotation,as indicated by arrows 51, and in the same direction as the localcirculation within each compartment 34, 36, 38, as designated by thearrows 32.

It should be noted at this point that it is important that openings 52be high enough and the raised elbownozzles 50 be at a proper level tocause the high concentration of surface oil to migrate from onecompartment to another in a continuous and uniform pattern, therebypreventing the concentration of oil in any one location and thedepletion of oil in any other location. In substance, the totalcirculation pattern provides a uniform covering of oil over the entirebody of liquid refrigerant within the unit.

In order to get most effective tube wetting with minimum refrigerantlevel, it is important that the boiling action of the incomingrefrigerant be confined to the specific tube bundles or vertical tubecolumns 24, and not be permitted to escape up through the pass ribchannels 54, outer edges 56, or center channel 58, see FIG- URE 1. Thechannels 54 are present as a result of partitions required inconventional headers (not shown) provided at each end of the evaporator.To make certain that this does not happen, blocks 60 are insertedbetween the vertical tube columns 24 at the tops of channels 54.Further, a cap member 62 is provided over the center channel 58. Stillfurther, blocking members 64 are provided along the sides of the tubebundle 20, as indicated in FIGURE 1.

The specific design of the block 60, cap member 62 annd block 64 is notcritical. However, the function they perform is highly important, namelyto prevent vapor from escaping up through any open channels, and thus,the blocks are inserted near the top of the channels. It will beappreciated that if the open channels were not blocked, they wouldfunction much in the manner of a chimney. A low pressure condition wouldthus result at the bottom of the channels. This low pressure conditionwould accelerate vaporization of refrigerant directly beneath thechannels. The splashing action caused by this vaporization would thusoccur directly beneath the channels and result in a large portion of thesplashed refrigerant being directed up the channels rather than throughthe tubes. It is desired to have splashing occur beneath the tubes sothat the tubes will be wetted. By preventing the occurrence of lowpressure conditions at the bottom of the open channels, vaporization ofre frigerant will take place uniformly over the entire pool ofrefrigerant thus resulting in flashing of liquid re= frigerant evenly onthe tubes. I

In FIGURE 1, there is shown one method of com struction for achievingthe rectangular tube bundle re quired for optimum results, wherein theevaporator is in its own individual shell. In this arrangement, thetrans verse plate 16 is continuous, forming the required flat bottom ofthe evaporator proper for holding liquid refrigerant at a desireduniform depth. The underlying curved bottom 18 forms the bottom exteriorwall of the shell. As previously indicated, this is of concaveconfiguration and therefore resistant to internal pressures; I

A vapor space 66 is provided between the concave bot;- tom wall 18 andthe flat transverse bundle support wall 16 to provide a dead gas spacefor purposes of thermal insulation against inward heat transfer from theambient atmosphere. g

It will be noted that a vent tube 68, FIGURE 1, is connected at itslower end into an opening 70 in wall 16 and extends upwardly above thetop level of the tube bundle 20. It will be noted that the top 72 ofvent tube 68 extends well above the top oftube bundle 20 and thus thevapor space 66 below wall 16 is vented to the upper vapor space 74 abovewall1-6 and the tube bundle, thereby equalizing pressures on theopposite sides of wall 16 and preventing any lateral load tendency tobuckle the same.

Bottom wall 18 thus works on pure tension or com pression to maintainthe shell shape when portions 12 and 16 are subjected to internal orexternal pressures; Because the vapor space 66 is filled withrefrigerant gas at very low pressure, it acts as an insulator so that noexternal insulation is required over the bottom of the shell, thuscontributing to manufacturing economy and operating efficiency of theunit. v

Vapor outlet for the unit is provided at 78.

When it is understood that stagnant gaseous refriger= ant, either hot orcold, is a good heat insulator, the econ= omy of the present inventionis fully appreciated. Be= cause the vapor space 66 is horizontal andnarrow, con vection currents are kept to a low level.

Also, by using a fairly wide structure with concave strengthening member18 beneath a structure is provided which permits the use of a shallowevaporator tube bun= die. This tube bundle configuration greatlyimproves the heat transfer coeflicient of the evaporator.

It has been found that a desired height for space 66,- as indicated atthe point 76, is about one inch per foot of concavity. Thus, for a threefoot span, a dimen sion at the point 76 would be about 1 /2 to about 3inches. This assures that convection and eddy currents are kept to aminimum, thus substantially neutralizing heat transfer to the interiorof the unit from ambient surroundings.

In accordance with the present invention, use is made of the addition ofsmall quantities of oil to the refrigerant to markedly improve theboiling action, the action continuing to wet all tubes even with liquidrefrigerant at a low load level and with the refrigerant at lower levelsin the bundle.

As is well known, in the prior art, the strongest boiling action occursat the entering water, i.e., where the water temperature is highest.This causes the level of refrigerant to drop in this region of anevaporator and thus effects a gradient of surface flow from the otherareas of the evaporator to this warmest area. Because the fresh, cleanrefrigerant from the condenser is being brought in at the bottom andbecause of the boiling action, the oil is concentrated near the top ofthe refrigerant. Thus, the surface flow mentioned above carries the oilwith it and the oil concentrates at the warm-' est area of theevaporator and the other areas become essentially oil free. This isobviously undesirable be cause it negates the effect of the oil in themajor part of the evaporator and amplifies the natural difference inboiling action due to difference in water temperature.

The present invention, by utilizing a uniform refrigerant level by theaforedisclosed distribution and circulation system, distributes theliquid refrigerant in such a way that oil distribution is maintaineduniform as a layer over the entire body of liquid and the tendency forit to concentrate is therefore counteracted.

Although the foregoing description has related to a unitary refrigerantevaporator for use by itself; that is physically separated from otherrefrigeration equipment except for connecting lines, it is to beincluded within the scope of invention to utilize the novel evaporatorstructure in a unitary shell, as associated with a condenser, alsopositioned therein. When so operating, the evaporator structure isdesirably placed uppermost in the shell structure, with advantagethereby being taken of the gas space 66 for insulation purposes toprevent heat transfer between the two associated units.

The foregoing description has related to the wall 18 as being of concaveconfiguration. However, the broad scope of invention would include aflat wall at 18, given sufficient subjacent support as by tube bundlesupports of a subjacent condenser of a unitary shell structure mentionedabove.

If desired, the bottom wall 13 may be insulated in some applicationswhere heat transfer must be kept to an absolute nil level.

From the foregoing, it will be evident that the present inventionprovides a unique and more highly eficient evaporator tube bundlearrangement and liquid refrigerant distribution system than hasheretofore being provided in the prior art.

A further advantage of the present invention resides in the fact thatgreatest utilization of oil for tube wetting ancl for uniform boilingaction and heat transfer efficiency is provided by the novel and uniformrefrigerant distribution system of the present invention.

It will be noted that another advantage of the present invention residesnot only in improved heat transfer and efi'iciency of cooling even atlow liquid refrigerant levels, but also in the low head room provided byvirtue of the horizontal disposition of the device. Thus, low head roomand low installation costs are evident from the present invention.

Having thus described my invention, I claim:

1. A method of evaporating liquid refrigerant in a refrigerationevaporator system of the semi-flooded type in which the evaporatorincludes an elongated hollow shell having therein an axially extendingtube bundle partially submerged in a pool of liquid refrigerant andwherein the boiling action of the refrigerant splashes liquid from alower level over the upper tubes characterized in the steps of formingthe liquid refrigerant into at least one horizontally disposed pool ofuniform depth, moving the refrigerant from a point of higher pressurethan the pressure in the evaporator into said pool in a plurality ofstreams flowing parallel to the bottom thereof and directed in ahorizontal fan pattern which produces a swirling motion in the poolabout a horizontal circle and uniformly distributes the incomingrefrigerant throughout the pool, thereby resulting in flash vaporizationof a portion of the incoming refrigerant due to the pressuredifferential whereby, as a result of the uniform distribution of theincoming refrigerant, the flash vaporization occurs uniformly throughoutthe pool, and heat exchanging the liquid refrigerant with warmer fluidpassing through the tube bundle.

2. A method according to claim 1 characterized in that the streams ofliquid refrigerant originate along the longitudinal axis of the pool andare directed outwardly therefrom towards the outer edges of the pool.

3. A method of evaporating liquid refrigerant in a refrigerationevaporator system of the semi-flooded type in which the evaporatorincludes an elongated hollow shell having therein an axially extendingtube bundle partially submerged in a pool of liquid refrigerant andwherein the boiling action of the refrigerant splashes liquid from alower level over the upper tubes, characterized in the steps of formingthe liquid refrigerant into at least one horizontally disposed pool ofuniform depth, providing a layer of oil overlying the pool ofrefrigerant, moving streams of liquid refrigerant into said pool,directing said streams of liquid refrigerant parallel to the bottom ofsaid pool in a horizontal fan pattern which uniformly distributes theincoming refrigerant throughout the pool and causes continuoushorizontal swirling movement of the entire pool of refrigerant to resultin continuous migration of the oil to maintain the oil as a uniformlayer, moving the refrigerant from a point of higher pressure than thepressure in the evaporator to result in flash vaporization of a portionof the incoming refrigerant due to the pressure differential whereby, asthe result of the uniform distribution of the incoming refrigerant, theflash vaporization occurs uniformly throughout the pool, and heatexchanging the liquid refrigerant with warmer fluid passing through thetube bundle.

4. The method according to claim 3, characterized in the step of formingthe liquid refrigerant into a plurality of horizontally disposed poolsabutted at their ends and open to one another at spaced points alongtheir ends, aiming the streams of liquid refrigerant to cause continuouscirculatory movement of each individual pool of liquid refrigerant andcontinuous circulatory movement between adjacent pools of liquidrefrigerant to maintain the oil as a uniform layer.

References Cited by the Examiner UNITED STATES PATENTS 1,937,802 12/1933Baer 62527 X 2,022,787 12/1935 Smith 62114 X 2,059,725 11/1936 Carrier-108 X 2,114,128 4/1938 Smith 62-502 2,147,788 2/1939 Gay 623942,189,731 2/1940 Hanson 62310 2,440,930 5/1948 Camilli et a1. 62502 X2,854,828 10/1958 Garland 62527 X 3,210,955 10/1965 Anderson et al.62l97 X OTHER REFERENCES Modern Refrigeration and Air Conditioning,Althouse and Turnquist, 1956, page 463 relied on.

LLOYD L. KING, Primary Examiner.

1. A METHOD OF EVAPORATING LIQUID REFRIGERANT IN A REFRIGERATIONEVAPORATOR SYSTEM OF THE SEMI-FLOODED TYPE IN WHICH THE EVAPORATORINCLUDES AN ELONGATED HOLLOW SHELL HAVING THEREIN AN AXIALLY EXTENDINGTUBE BUNDLE PARTIALLY SUBMERGED IN A POOL OF LIQUID REFRIGERANT ANDWHEREIN THE BOILING ACTION OF THE REFRIGERANT SPLASHES LIQUID FROM ALOWER LEVEL OVER THE UPPER TUBES CHARACTERIZED IN THE STEPS OF FORMINGTHE LIQUID REFRIGERANT DEPTH, AT LEAST ONE HORIZONTALLY DISPOSED POOL OFUNIFORM DEPTH, MOVING THE REFRIGERANT FROM A POINT OF HIGHER PRESSURETHAN THE PRESSURE IN THE EVAPORATOR INTO SAID POOL IN A PLURALITY OFSTREAMS FLOWING PARALLEL TO THE BOTTOM THEREOF AND DIRECTED IN AHORIZONTAL FAN PATTERN WHICH PRODUCES A SWIRLING MOTION IN THE POOLABOUT A HORIZONTAL CIRCLE AND UNIFORMLY DISTRIBUTES THE INCOMINGREFRIGERANT THROUGHT THE POOL, THEREBY RESULTING IN FLASH VAPORIZATIONOF A PORTION OF THE INCOMING REFRIGERANT DUE TO THE PRESSUREDIFFERENTIAL WHEREBY, AS A RESULT OF THE UNI-